Visual System (CN II) Made by : DANI MAMO

Visual pathway The retina is the receptor surface for visual information. Like the optic nerve, it is a portion of the brain, despite its physical location at the periphery of the central nervous system. Its most important components are the sensory receptor cells, or photoreceptors, and several types of neurons of the visual pathway. The deepest cellular layer of the retina contains the photoreceptors (rods and cones); the two more superficial layers contain the bipolar neurons and the ganglion cells.

Rods and cones When light falls on the retina, it induces a photochemical reaction in the rods and cones, which leads to the generation of impulses that are ultimately propagated to the visual cortex. The rods were long thought to be responsible for the perception of brightness and for vision in dim light, while the cones were thought to subserve color perception and vision in bright light. More recent research, however, has cast doubt on these hypotheses. The underlying mechanisms of these processes are probably much more complex but cannot be discussed here in any further detail.

The fovea is the site of sharpest vision in the retina and contains only cones, which project onto the bipolar cells of the next neuronal layer in a one-to-one relationship. The remainder of the retina contains a mixture of rods and cones. The retinal image of a visually perceived object is upside-down and with left and right inverted, just like the image on the film in a camera.

Optic nerve, chiasm, and tract The retinal bipolar cells receive input onto their dendrites from the rods and cones and transmit impulses further centrally to the ganglion cell layer. The long axons of the ganglion cells pass through the optic papilla (disk) and leave the eye as the optic nerve, which contains about 1 million fibers. Half of these fibers decussate in the optic chiasm: the fibers from the temporal half of each retina remain uncrossed, while those from the nasal half of each retina cross to the opposite side

Thus, at positions distal (posterior) to the optic chiasm, fibers from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina are united in the optic tract. A small contingent of optic nerve fibers branches off the optic tracts and travels to the superior colliculi and to nuclei in the pretectal area. These fibers constitute the afferent arm of various visual reflexes, and, in particular, of the important pupillary light reflex

Lateral geniculate body, optic radiation, and visual cortex The optic tract terminates in the lateral geniculate body, which contains six cellular layers.Most of the optic tract fibers end here, forming synapses with lateral geniculate neurons. These, in turn, emit fibers that run in the hindmost portion of the internal capsule and then form a broad band that courses around the temporal and occipital horns of the lateral ventricle, the so-called optic radiation of Gratiolet. The fibers of the optic radiation terminate in the visual cortex, which is located on the medial surface of the occipital lobe, within, above, and below the calcarine fissure (Brodmann area 17).

Fibers derived from the macula occupy the largest area of the visual cortex. Area 17 is also known as the striate cortex because it contains the stripe of Gennari, a white band composed of horizontally running fibers, which can be seen with the naked eye in sectioned anatomical specimens.

Somatotopic organization of the visual pathway Although the fibers of the visual pathway partially decussate in the optic chiasm, a strict point-to-point somatotopic organization of the individual nerve fibers is preserved all theway from the retina to the visual cortex.

Visual information is transmittedcentrally as follows Visual information is transmittedcentrally as follows.Anobject locatedin the left visual field gives rise to images on the nasal half of the left retina and the temporal half of the right retina.Optic nerve fibers derived fromthe nasal half of the left retina cross to the left side in the optic chiasm to join the fibers from the temporal half of the right retina in the rightoptic tract. These fibers then pass to a relay station in the right lateral geniculate body, and then by way of the right optic radiation into the right visual cortex. The right visual cortex is thus responsible for the perception of objects in the left visual field; in analogous fashion, all visual impulses relating to the right visual field are transmitted through the left optic tract and radiation into the left visual cortex

Visual fibers derived from the macula are found in the temporal portion of the optic disk and in the central portion of the optic nerve .Damage to these fibers can be seen by ophthalmoscopy as atrophy of the temporal portion of the disk (temporal pallor).